A way to preserve free-vars and related annotations between different passes of the compiler.

[Idorobots]

Currently the annotate-free-vars pass adds free variable metadata to the AST, but that information is gradually lost when the other passes run. It's inconvenient to always maintain these (for instance reorder-letrec-bindings) but it is really inconvenient that these are not preserved. It's also slow to recompute on each AST change.
Perhaps keeping these annotations in sync could be extracted into a generic AST walker and intertwined with the other passes?

I.e.

(walk-ast (with-free-vars
           (lambda (expr)
             ...
             ))
          expr)

...ensures that output of the pass maintains all the free vars correctly.